The PT79SR100 requires an over-voltage protection (OVP) zener diode on the output to prevent turn-on overshoot. The table below lists the appropriate zener voltage and suggested zener diode part number. We recommend the 1N53xxB series because of its surge current rating.

This application note describes the problem of an inrush current of the DC brushless fan during start up, and explains how this undesirable huge inrush current is lowered to be within the acceptable limits by using TI's TPS25924 e-Fuse. Bringing inrush current within safe limits helps the designer to achieve a lowcost solution.

Brushless DC fans have been an effective and easy method of cooling power electronic circuits. Brushless DC fans having the advantage of small size and high reliability are used extensively in many new applications. Because the speed and airflow of a typical DC fan is proportional to the voltage supplied, a single product can be used to meet the different applications by setting the supply voltage to provide the desired airflow. Brushless DC fans do not draw constant DC currents even under a steady state condition.

When designing the electronics that interface to a DC brushless cooling fan, it is critically important to be aware that brushless cooling fans draw substantially higher inrush current than steady state running current. The choice of the input power source and energy storage to support the inrush current drawn is affected by the type of DC fans and its motor current characteristics. To meet the large inrush current, demand of the cooling fan designer is generally left with choices such as using the oversized DC power supply which does not cause the voltage drop while the fan draws the inrush current, and selecting a driver that can safely handle the large inrush current or installing adequate current limiting – which is not the desired solution from a system cost and reliability view point.